A tool holder is clamped in the spindle of a machine tool. The structure for clamping the tool holder therein is as shown in FIG. 6.
In this figure, 1 points out a spindle body rotatably supported in the spindle head. A tapered hole 1a into which the tapered section 2a of the tool holder 2 is inserted is formed at the tip end. Thereof, and an operating hole 1b on which a staged portion "m" is formed at the upper part thereof is provided. 3 points out a sleeve which is inserted in the corresponding operating hole 1b and is engaged with at the position of the staged portion "m", and the diameter of the lower portion of the inner hole 3a thereof is set to a larger value than that of the upper portion thereof. And a draw bar 4 is pulled upwards by elongation force of a coned disk spring 5 and is so arranged that it can be displaced downwards. The lower end 4a thereof is provided with a cylindrical hole 4c, and a plurality of through holes 4b are provided in the direction of radius with a proper interval in the inward direction at the inlet side thereof. Balls 6 are inserted in the through holes 4b and are so composed that they can engage with the pull stud 2b of the tool holder 2.
In such a structure as shown in the above, when clamping the tool holder 2, the draw bar 4 is displaced toward the downward direction as shown with an imaginary line "n" in the figure against the elongation force of the coned disk spring 5. Under this condition, the tool holder 2 is inserted from downwards toward inwards of the tapered hole 1a of the spindle 1, thereby causing the pull stud 2b of the tool holder 2 to arrive at the solid line position in the figure, excluding the ball 6 outwards of the through holes 4b. Thereafter, the draw bar 4 is made free and is displaced upwards by elongation force of the coned disk spring 5, thereby causing the ball 6 to move inwardly in the through holes 4b with the outer surface thereof guided in the inner hole 3a of the sleeve 3 and to reach the solid line position in the figure where the pull stud 2b is engaged and fixed. Thereafter, this condition is continuously maintained.
To the contrary, when unclamping the tool holder 2, the draw bar 4 is displaced toward the downward direction as shown with an imaginary line "n" in the figure against the elongation force of the coned disk spring 5 as mentioned above, thereby causing the tool holder 2 to be released.
In order to acquire a strong force by the conventional structure, it is necessary to increase the elongation force of the coned disk spring 5 according to the increment of the clamping force.
However, increasing the elongation force of the coned disk spring 5 results in increase of the operating force of the draw bar 4 when clamping and unclamping the tool holder 2, and increase of the corresponding operating force further derives a new problem, that is, increase of the thrust of bearings by which the spindle 1 is supported.
Under such a circumstance, a spindle structure by which the tool holder 2 is strongly clamped to the spindle 1 without excessively increasing the elongation force of the coned disk spring 5 is much desired.
Also, if the draw bar 4 is displaced downwards in order to unclamp the tool holder 2 with the conventional structure, the tool holder 2 is naturally dropped from the spindle 1 by gravity action. For this reason, there is a limitation in use, that is, it is not possible to clamp and unclamp the draw bar 4 unless the tool holder is securely grasped by the tool holder holding mechanism of the tool change arm when changing tools by means of an automatic tool changer, thereby causing such a problem as tools can not be quickly changed, to occur.